The present invention relates to improvements in interfacing video displays with video sources, computers and the like, in which non-standard sync signals may typically be used.
Computers, arcade games and the like typically generate vertical sync and horizontal sync signals as separate internal signals which are then transmitted to the appropriate display apparatus. However, sending the horizontal and vertical sync as separate signals requires at least two transmission lines with the resultant extra cost and hardware. Accordingly, it is desirable to transmit all sync information over a single line.
In a standard television system utilizing composite video signals, both horizontal and vertical sync information is superimposed on the video information signal. At the receiving station, the sync information may be extracted in the form of a single, composite sync pulse train bearing both horizontal and vertical sync information. During a vertical sync period and the corresponding vertical retrace, the horizontal timing information is shifted in phase. Accordingly, the horizontal oscillator circuit will typically shift in phase during this period. At the end of the vertical sync period, the horizontal oscillator circuit will reestablish its original timing and lock onto the now correctly phased horizontal sync pulses.
Vertical and horizontal sync typically have been transmitted separately in computer applications so that during the vertical sync period, the horizontal flyback circuitry may keep the video raster scan correctly timed and preserve the picture integrity during the first few horizontal lines at the top of the screen. This is especially desirable in computer graphics, text, and game display applications, because the first few lines may typically contain important information for display. Unlike traditional television systems where a vertical blanking interval may hide any disturbances caused by the vertical sync, in a computer display environment it is desirable to use all available horizontal lines for display.
Thus, although it is desirable to provide vertical and horizontal sync on a single transmission line, it is undesirable to lose or alter the horizontal sync information during the vertical sync interval because of the resultant "recovery" time necessary for the horizontal flyback circuitry to reestablish accurate horizontal timing once the vertical sync period terminates. Accordingly, it is desirable to provide a technique for allowing single line transmission of both horizontal and vertical sync while minimizing the disturbance to horizontal timing introduced by the presence of the vertical sync signal.
Because both the horizontal and the vertical sync signals may be thought of as essentially digital signals with a horizontal or vertical pulse comprising a "1" (or a "0" if negative sync is utilized) and the period between pulses comprising a "0" (or a "1" if negative sync is utilized), standard logic circuitry may be used to combine the two sync signals for transmission on a single line. However, sync information may become lost through such a combination and the sync periods may lose some definition through this means. Ultimately, a combination of the two sync signals through standard logic circuitry may require additional horizontal lines to accomplish the vertical retrace and reestablish the correct timing of the horizontal flyback, which is undesirable in a computer or graphics display system in which every line is valuable.
Accordingly, it is a principal object of the present invention to provide a technique which generally overcomes the deficiencies in the prior art.
It is a primary object of the present invention to provide a technique for summation sync which allows the horizontal and vertical sync information to be transmitted on a single line.
It is still a further object of the present invention to provide single line transmission of horizontal and vertical sync information and minimization of disturbance to the horizontal timing caused by the vertical sync signal.
It is still a further object of the present invention to provide an apparatus and method for implementing the technique of the present invention.